1. Field of the Invention
The present invention relates to a semiconductor device of a CMOS structure having a fine contact and a method of manufacturing the same.
2. Description of the Related Art
Decrease of parasitic resistance becomes a larger problem as miniaturization of the semiconductor device is enhanced. However, in the fine contact, it has been difficult to reduce the contact resistance in accordance with the scaling.
In order to obtain a good contact to a semiconductor substrate in a bottom surface of the contact, titanium silicide has been conventionally formed on the bottom surface of a contact hole by sputtering, plasma CVD and the like (pages 5 and 6 and FIGS. 10 and 12 of Jpn. Pat. Appln. KOKAI Publication No. 11-214650, and pages 6 and 7 and FIG. 7 of Jpn. Pat. Appln. KOKAI Publication No. 2001-250792). This treatment deoxidizes a native oxide film on a surface of the semiconductor substrate in which the bottom surface of the contact hole is present. The native oxide film forms most part of the contact resistance. However, it is not sufficient in order to further decrease of the contact resistance.
Generally, a relationship between a work function φm of a metal and a work function φs of a semiconductor determines which of Schottky junction and ohmic junction is created at an interface between the metal and semiconductor. For example, when an n-type semiconductor is in contact with a metal, the metal/semiconductor interface has the Schottky junction if φm>φs, while the metal/semiconductor interface has the ohmic contact if φm<φs. On the other hand, when a p-type semiconductor is in contact with a metal, the metal/semiconductor interface has the ohmic junction if φm>φs, while the metal/semiconductor interface has the Schottky contact if φm<φs. The work function φs of the semiconductor is changed by an impurity concentration, the work function φs is in the range of about 4.0 eV to 4.2 eV in the case of the n-type semiconductor, and the work function φs is in the range of about 4.9 eV to 5.1 eV in the case of the p-type semiconductor. Therefore, in the case that a metal is in contact with both the n-type semiconductor and the p-type semiconductor, one of the contacts has the ohmic junction and the other contact has the Schottky junction.
A metal has been commonly used in forming both a contact to an n-type semiconductor and a contact to a p-type semiconductor. Since the work function of titanium silicide, which is usually used, is 4.1 eV, then the contact has the ohmic contact to the n-type semiconductor, while the contact has the Schottky junction (i.e., non-ohmic contact) to the p-type semiconductor. Accordingly, the contact resistance on the p-type semiconductor region (i.e., the resistance between the titanium silicide and the p-type semiconductor region) becomes higher than that on the n-type semiconductor region (i.e., the resistance between the titanium silicide and the n-type semiconductor region).
As a matter of course, in order to decrease the contact resistance, it is desirable that both of the contacts have the ohmic junction. To attain such contacts, for example, a contact hole to the n-type semiconductor is formed, and then a contact layer of metal A satisfying the relationship of φm<φs is formed in the contact hole. Further, a contact hole to the p-type semiconductor is formed, and then a contact layer of metal B satisfying the relationship of φm>φs is formed in the contact hole. However, in order to realize such a forming method, it is necessary to establish a technique including, e.g. the sputtering, plasma CVD technique and the like, for embedding at least two kinds of metals into the inside of the fine contact. Further, two processes are required to be carried out to form the contacts, and thus the number of processes required increases. Accordingly, it has not been easy to form a contact having the ohmic junction to the n-type semiconductor and the p-type semiconductor.